1. Field of the Invention
The disclosed embodiments of the present invention relate to reading data stored in a flash memory, and more particularly, to a method and memory controller for reading data stored in a flash memory by referring to binary digit distribution characteristics of bit sequences read from memory cells of the flash memory.
2. Description of the Prior Art
Flash memory can be electrically erased and programmed for data storage. It is widely used in memory cards, solid-state drives, portable multimedia players, etc. As the flash memory is a non-volatile memory, no power is needed to maintain the information stored in the flash memory. Besides, the flash memory offers fast read access and better shock resistance. These characteristics explain the popularity of the flash memory.
The flash memories may be categorized into NOR-type flash memories and NAND-type flash memories. Regarding the NAND flash memory, it has reduced erasing and programming time and requires less chip area per cell, thus allowing greater storage density and lower cost per bit than the NOR flash memory. In general, the flash memory stores data in an array of memory cells made from floating-gate transistors. Each memory cell can store one bit of information or more than one bit of information by adequately controlling the number of electrical charge on its floating gate to configure the threshold voltage required for turning on the memory cell made of a floating-gate transistor. In this way, when one or more predetermined control gate voltages are applied to a control gate of the floating-gate transistor, the conductive status of the floating-gate transistor would indicate the binary digit(s) stored by the floating-gate transistor.
However, due to certain factors, the number of electrical charges originally stored on one flash memory cell may be affected or disturbed. For example, the interference presented in the flash memory may be originated from write (program) disturbance, read disturbance, and/or retention disturbance. Taking a NAND flash memory including memory cells each storing more than one bit of information for example, one physical page includes multiple logical pages, and each of the logical pages is read by using one or more control gate voltages. For instance, regarding one flash memory cell which is configured to store three bits of information, the flash memory cell may have one of eight possible states (i.e., electrical charge levels) corresponding to different electrical charge amounts (i.e., different threshold voltages), respectively. However, due to the increase of the program/erase (P/E) count and/or the retention time, the threshold voltage distribution of memory cells in the flash memory may be changed. Thus, using original control gate voltage setting (i.e., threshold voltage setting) to read the stored bits from the memory cell may fail to obtain the correct stored information due to the changed threshold voltage distribution.